SPACE-AIR-GROUND INTEGRATED
NETWORKS WITH FSO
COMMUNICATION
Swaminathan R
Assistant Professor
Department of Electrical Engineering
Indian Institute of Technology Indore
OUTLINE
 Introduction:
Free Space Optics (FSO) Communication
o Advantages and applications of FSO
o Drawback of FSO
o Schemes to improve FSO
 Aerial
 6G
Platforms : Use cases and advantages
Communications
 Space-Air-Ground
Integrated Networks
2
INTRODUCTION: FREE SPACE OPTICS
•
RF communication based on the electromagnetic spectrum is capacity limited
and is also costly as most of the frequency bands are licensed
•
Demand for RF spectrum over microwave band is also increasing with the
growing popularity of new wireless devices
•
Data heavy wireless devices demand other possible options for wireless
communications
•
Growing interest has been witnessed in the research and development of optical
wireless communication in infrared, visible light, and ultra violet frequency bands
•
FSO communication, term used for OWC in outdoor scenario, refers to a LoS
communication between two fixed points in the atmosphere using optical carrier
•
Supports high data rate up to GBPS for short-range transmission up to few
kilometers due to the availability of license free optical carrier frequencies (THz)
FSO FOR SATCOM
• World Disaster Report 2019 [8]: 396 natural disasters have been recorded in 2019
with more than 95 million people affected across the globe
• Note: More than 40 percent of the total number of disasters occurred in Asia
• Important for India and other Asian countries to deploy more satellites and increase the
monitoring frequency, observation area, and resolution of images obtained from the
satellite.
• There is a need for large communication capacity and reliable communication links in
SATCOM systems
• Most of the SATCOM systems are based on RF links with data rate limited to few hundred
Mbps
• Growing interest has been witnessed in R & D of establishing FSO links between ground
station (GS) and satellite that can offer much larger data rates up to several Gbps
[8] Natural disasters 2019: Now is the time to not give up Centre Res. Epidemiology Disasters, Brussels, Belgium, 2020. [Online]. Available: https://www.emdat.be/publications
INTRODUCTION: FREE SPACE OPTICS
•
FSO transmission in unguided media using optical carriers [1]
•
FSO systems use narrow laser beams for data transmission
• Operates at frequencies above 100 GHz near IR band in EM spectrum.
• FSO wavelength range: 700nm – 1600nm, Bandwidth: From 100 GHz to few THz
• RF wavelength range: 30mm – 3m, Bandwidth: of the order of MHz and max. 1 GHz
• Free space optics (FSO) is an emerging alternative to RF systems
Image source: [1] M. A. Khalighi and M. Uysal, "Survey on Free Space Optical Communication: A Communication Theory Perspective," in IEEE Communications Surveys & Tutorials, vol. 16,
no. 4, pp. 2231-2258, 2014.
ADVANTAGES OF FSO
• Data rate advantage over RF : Provides high
bandwidth and high data rates (upto 10 Gbps)
• Unlicensed spectrum :
o No license fees required for FSO
communication
o Current radio frequency (RF) communication is
licensed and congested
• Spatial confinement: Provides high reuse
factor, security and susceptible to interference
• Ease of deployment : Cheaper flexible
connectivity without needing to dig, lay and
maintain underground cables
Optical Fiber
FSO
Image source:
https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Map%3A_Introductory_Chemistry_(Tro)/09%3A_Electrons_in_Atoms_and_the_Periodic_Table/9.03%3A_The_Electromagnetic_Spectrum
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APPLICATIONS OF FSO
• Video surveillance and monitoring:
high quality video transmission
• Intercampus connectivity: can
support ultra high speeds
• Backhauling applications: FSO
provides huge bandwidth requirements
• Broadcasting live events: FSO is
capable of providing high throughput
Image source: M. Z. Chowdhury, M. K. Hasan, M. Shahjalal, M. T. Hossan and Y. M. Jang, "Optical Wireless Hybrid Networks: Trends, Opportunities, Challenges,
and Research Directions," IEEE Commun. Surveys & Tuts., vol. 22, no. 2, pp. 930-966, Secondquarter 2020.
DRAWBACKS IN FSO
•
Atmospheric turbulence (fading/scintillation )
o Inhomogeneity in the atmospheric conditions.
o Abrupt change in temperature, pressure, and wind
leading to change in refractive index of air
•
Misalignment or pointing errors
o Due to random deflection of the beam
o Due to building sway and mechanical vibration
•
Atmospheric loss due to weather conditions
o Fog, dust, smoke, water droplets absorb laser light
8
F. Nadeem, V. Kvicera, M. S. Awan, E. Leitgeb, and S. S. Muhammed, and G. Kandus, “Weather effects on hybrid FSO/RF communication link,’’ IEEE Journal on selected areas in communications, vol. 27, no. 9, Dec. 2009.
SCHEMES TO IMPROVE FSO PERFORMANCE
• Spatial diversity
o Receiver diversity: Single Input Multiple Output (SIMO)
o Transmit diversity: Multiple Input Single Output (MISO)
o Multiple Input Multiple Output (MIMO) FSO system
• Relay assisted transmission: cooperative diversity
• Hybrid RF/FSO systems: back-up RF link
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UAV-ASSISTED FSO
•
Receiver needs to be placed within the LoS of the
transmitter in an urban environment with tall buildings
for backhaul networks
•
Ground-based relay nodes need to be deployed
between the transmitter and receiver of a backhaul
network for creating the LoS FSO link
•
Always not possible to obtain permission for deploying
ground-based relay nodes in optical locations
•
Optimal positions for deploying such ground relays
may not be always possible and hence, risk of
exposure at rental relay locations
•
So increase in the number of ground relays with
increase in implementation cost and decrease in
security at the rental relay locations
M.T. Dabiri, et.al “UAV-Assisted Free Space Optical Communication System with Amplify-and-Forward Relaying”, IEEE Trans. Veh. Technol..,
Early access, 2021.
UAV-ASSISTED FSO
• UAVs are more likely to have LoS connection with ground-based nodes as they are
situated well above the ground at a higher altitude and are adopted as aerial relays
• So UAV communication has been a key enabler for smart cities and future Internet of
Everything systems
• In summary, compared with the conventional terrestrial relays, UAV-based relays have the
intrinsic advantages of establishing the LoS links by adjusting positions dynamically
J.YWang, et.al “Hovering UAV-Based FSO Communications: Channel Modelling, Performance Analysis, and Parameter Optimization”, IEEE
JSAC.., Early access, 2021.
AERIAL PLATFORMS
▪
Aerial platforms such as unmanned-aerial-vehicles (UAVs), high-altitude
platform station (HAPS), etc. have been considered to play a promising role in
5G and beyond (5GB) and 6G wireless networks
▪
Space-air-ground integrated networks (SAGIN), have been receiving much
needed attention among researchers
▪
Use cases : Flying Aerial Base Station and Mobile Relay Station
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Image Source : www.ecagroup.com
ADVANTAGES
◦ LoS Links: Aerial platforms can connect to ground users via LoS (Line of Sight)
links, facilitating highly reliable transmissions over long distances
◦ Dynamic Deployment: They can be dynamically deployed according to real-time
requirements and allocated to different users or controllers on demand
◦Multi-aerial-platform networks offer ubiquitous connectivity to ground users
◦ UAVs can expand communication network in a fast and cost effective ways as
compared to fixed ground and satellite infrastructures
◦Medium operational costs, easy maintenance, and environmental friendliness, etc.
◦ Play a vital role in providing network service recovery especially in a disaster-prone
region for handling emergency situations
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USE CASES : UAV
UAV-aided Ubiquitous Coverage :
•
UAVs are deployed to assist the existing
communication infrastructure, if any, in
providing seamless wireless coverage
within the serving area
•
Two example scenarios : (a) Rapid
service recovery after partial or complete
infrastructure damage due to natural
disasters (b) Base station offloading in
extremely crowded areas (e.g., a
stadium during a sports event)
• Second case has been identified as one of the five key scenarios that need to be
effectively addressed by fifth generation (5G) and beyond wireless systems
Y. Zeng, R. Zhang and T. J. Lim, "Wireless communications with unmanned aerial vehicles: opportunities and challenges," in IEEE Communications
Magazine, vol. 54, no. 5, pp. 36-42, May 2016, doi: 10.1109/MCOM.2016.7470933.
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USE CASES : UAV
UAV-aided relaying :
•
UAVs are deployed to provide
wireless connectivity between two or
more distant users or user groups
without reliable direct communication
links.
•
For example, this could be between
the frontline and the command center
for emergency responses
UAV-aided information dissemination and data collection :
• UAVs are deployed to disseminate (or collect) information to (from) a large number of
distributed wireless devices.
• Example: Wireless sensors in precision agriculture applications
Y. Zeng, R. Zhang and T. J. Lim, "Wireless communications with unmanned aerial vehicles: opportunities and challenges," in IEEE Communications
Magazine, vol. 54, no. 5, pp. 36-42, May 2016, doi: 10.1109/MCOM.2016.7470933.
USE CASES : AERIAL PLATFORMS
Integrated Space-Air-Ground Networks
•
To achieve the demand of ubiquitous mobile ultra-broadband services in 6G communication, it
is important to integrate terrestrial, satellite, and airborne networks into a single wireless
system, which leads to the development of SAGIN
•
A scenario for multicast communication is shown, where a ground transmitter intends to send
information signals to multiple ground users.
6G COMMUNICATIONS
•
As fifth-generation (5G) wireless technologies are currently being rolled out
worldwide, an upsurge of interest has been witnessed recently in the research
and development of 6G wireless technologies.
•
It is expected that 6G wireless networks will have 1000 times higher
simultaneous wireless connectivity than 5G wireless networks
•
Further, it is also expected that 6G will include ubiquitous mobile ultrabroadband services everywhere including new environments such as sky,
sea, etc. compared to the enhanced mobile broadband (eMBB) services in 5G
•
FSO can support seamless ubiquitous high-speed broadband connectivity,
which is a significant requirement for 6G backhaul networks
6G COMMUNICATIONS
• In 6G networks, it is envisioned that data transmission will also be over vertical
heterogeneous networks, which is also known as space-air-ground integrated
networks (SAGIN)
• By integrating, we can achieve ubiquitous mobile ultra-broadband services in 6G
communication
• In order to meet the higher data requirements of SAGIN, FSO communication can be
employed between ground transmitter and satellite. Further, aerial platforms such as
HAPS can act as a relay station to enhance the reliability
• To serve ubiquitous IoT applications and services around the world, next-generation
communication technology will broaden coverage by integrating communications in
different spatial domains to form SAGIN
• SAGIN is a critical solution for expanding network coverage and improving network
performance. SAGIN will include a significant number of battery-powered IoT nodes
due to the broad geographic range and dynamics of free space
HAPS-BASED FSO FOR SATCOM
• HAPS is an aerial vehicle situated well above the clouds at typical heights of 17 to 25 km and the
atmospheric impact on an optical beam is less severe than directly above ground .
• Satellite-to-Ground station (GS) FSO uplink/downlink link performance degrades mainly due to
atmospheric turbulence
• HAPS can act as a relay station between satellite and GS and improve the link performance
• By deploying HAPS as a relay terminal, the satellite-to-GS downlink is split into two parts:
(i) Satellite-to-HAPS (ii) HAPs-to-GS
• HAPS is located in a cloud-free atmospheric altitude and reliability is not a major concern for Satelliteto-HAPS link
Problem: Link reliability or availability is a
key concern for HAPS-to-GS
Solution: Use an RF link as a back-up in the
case of FSO link outage
Reason: RF link is less sensitive to
atmospheric turbulence
Image source : Internet
SYSTEM MODELS
LEO Satellite – Source (S), HAPS – Relay (R), Ground Station – Destination
(D)
Two-phase transmission:
First Phase: S transmits to R
over FSO link
Second Phase: R transmits to
D over FSO or RF link
depending on the channel
conditions.
Switching Scheme:
𝑓
γ𝑅𝐷 > γ𝑡 - FSO link will be
used for transmission.
𝑓
γ𝑅𝐷 < γ𝑡 - Receiver sends a
1-bit feedback signal to
activate the RF sub-system
and the information will be
transmitted over RF link alone
(hard-switching) or MRC of
FSO and RF links (adaptive
combining)
• In the first use case, the
transmission is completed
through two HAPS by
using FSO communication
as a perfect LOS is
established between them
• In the second use case,
the
LOS
connectivity
cannot
be
formed
between two HAPS due to
the large distance and
stratospheric attenuation.
Thereby, an LEO satelliteaided optical cooperation
is created.
• In particular, G transmits
its
message
to
the
multicast group in fourtime slots over H1, The
LEO satellite S, and H2,
all working based on the
21
decode and forward (DF)
relaying protocol
REFERENCES
1.
2.
3.
4.
Deepshikha Singh and Swaminathan R, ``Comprehensive performance analysis of
hovering UAV-based FSO communication system," IEEE Photonics Journal, vol. 14,
no. 5, pp. 1-13, Oct. 2022
Swaminathan R, S. Sharma, N. Vishwakarma, and AS. Madhukumar, ``HAPS-based
relaying for integrated space-air-ground networks with hybrid FSO/RF
communication : A performance analysis", IEEE Transactions on Aerospace and
Electronic Systems, vol. 57, no. 3, pp. 1581-1599, June 2021.
S. Shah, M. Siddharth, N. Vishwakarma, Swaminathan R, and A. S. Madhukumar,
``Adaptive-combining-based hybrid FSO/RF satellite communication with and
without HAPS,'' IEEE Access, vol. 9, pp. 81492-81511, 2021.
O. B. Yahia, E. Erdogan and G. K. Kurt, "HAPS-Assisted Hybrid RF-FSO Multicast
Communications: Error and Outage Analysis," IEEE Transactions on Aerospace and
Electronic Systems, vol. 59, no. 1, pp. 140-152, Feb. 2023.
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